A high-temperature gas cooled reactor is a nuclear reactor which can take out a gas such as a helium gas with peculiar high safety and very high outlet temperature because a reactor core structure containing a nuclear fuel is formed from graphite having large calorific capacity and good high temperature soundness and also a gas such as a helium gas which never causes a chemical reaction under high temperature is used as a coolant gas. Thus, the high temperature heat of about 900° C. from the high-temperature gas cooled reactor can be used in broad fields such as a hydrogen manufacture and a chemical plant as well as an electric power generation.
(Coated Fuel Particles)
In general, coated fuel particles of about 500 micrometer to 1000 micrometer diameter are used as the nuclear fuel for this high-temperature gas cooled reactor. The coated fuel particles are formed by coating with total four layers of first to fourth layers, the fuel kernels of about 350 micrometer to 650 micrometer diameter obtained by sintering a uranium dioxide, a thorium, etc. to the form of ceramics. Specifically, the coated layers are the following four layers.
That is, the innermost first layer generally called a buffer layer is a layer which is formed of low-density thermal decomposition carbon (PyC) of a density of about 1 g/cm3 and serves to store a gas of a gas-like fission product (FP) and also to absorb swelling of the nuclear fuel. In general, the second layer applied onto this first layer is an inner thermal decomposition carbon (PyC) layer formed of high-density thermal decomposition carbon of a density of about 1.8 g/cm3 and serves to hold a gas-like fission product (FP) as a barrier of diffusion of the gas-like fission product (FP). The third layer called a silicon carbide (SiC) layer is formed of silicon carbide of a density of about 3.2 g/cm3 and serves to hold a solid-like fission product as a barrier of diffusion of the solid-like fission product and also serves as a main reinforcing member for the whole coated fuel particles. The outermost thermal decomposition carbon layer as the fourth layer is formed of high-density thermal decomposition carbon of a density of about 1.8 g/cm3 in the same manner as the second layer and serves to hold the strength of the whole coated fuel particles under irradiation by generating compression stress on the third silicon carbide layer by irradiation contraction and also to hold the gas-like fission product (FP).
Such coated fuel particles are generally manufactured through the following processes. First, concretely explaining the production of the fuel kernel, a dropping undiluted solution is produced by adding and agitating pure water and a thickening agent to a uranyl nitrate undiluted solution formed by melting uranium oxide powder in a nitric acid. In this case, the thickening agent is added so that the liquid drop of the dropped uranyl nitrate undiluted solution becomes true ball-like form by its own surface tension during its dropping. A resin such as a polyvinyl alcohol resin which has a property of being solidified under alkali conditions, polyethylene glycols and metolose for example may be used as this thickening agent. Subsequently, after cooling the dropping undiluted solution prepared in such a way to a predetermined temperature and adjusting its viscosity, it is dropped into the ammonia solution by vibrating a dropping nozzle of thin diameter. In this case, the deformation of the liquid drop is prevented when it lands on the ammonia solution surface by blowing an ammonia gas upon the liquid drop in space where it drops until it lands there so as to gel the surface of the liquid drop.
The undiluted solution dropped into the ammonia solution gets particles of heavy uranium acid ammonium by the full reaction of the uranyl nitrate with the ammonia in the ammonia solution. The particles of heavy uranium acid ammonium are roasted in the atmosphere to form uranium trioxide particles, which are further reduced and sintered to obtain the fuel kernels formed of high-density ceramics-like uranium dioxide. Since the diameter and the deviation from the spherical form of the thus obtained fuel kernels very substantially effect on the manufacture conditions in the subsequent coating process, the fuel kernels are fed to the coating process after their diameter is sorted by a sieve and their deviation from the spherical form is also sorted.
Thereafter, in the coating process of the fuel kernels, the fuel kernels are loaded in a fluid bed and sequentially coated with the first through fourth layers by thermally decomposing the coating gases. In this case, the first low-density carbon layer is applied onto the fuel kernels by thermally decomposing an acetylene (C2H2) at about 1400° C. The second and fourth high-density thermal decomposition carbon layers are applied by thermally decomposing a propylene (C3H6) at about 1400° C. The third silicon carbide layer is formed by thermally decomposing a methyl-chorolosilane (CH3SiCl3) at about 1600° C. The thus manufactured coated fuel particles get over-coated particles by further applying graphite matrix material comprising graphite powder, a caking agent, etc. on the surface of the coated fuel particles.
(Fuel Compact)
In using the thus over-coated coated fuel particle as a fuel compact, after dispersing the coated fuel particles in a graphite matrix material, they are molded by press or by die into a solid type or a hollow type cylindrical body and then sintered to produce the fuel compact 10 of predetermined form shown in FIG. 7(A) (see JP 2000-284084A, for example). This fuel compact 10 is formed by integrally binding a plural of coated fuel particles 12 by softening a phenol resin contained in the graphite matrix material by heating dies or punches when the coated fuel particles 12 are compressed as shown in FIG. 8.
(Loaded into a Reactor Core)
The thus formed fuel compact 10 has two kinds of solid type cylindrical body and hollow type cylindrical body and, in either case, 1) a predetermined amount of fuel compacts are contained in a fuel sleeve (cylinder) of graphite with its top and bottom closed by plugs so as to form a fuel rod and the fuel rods are loaded directly into a plural of insertion openings of a hexagon pillar type graphite block of the high-temperature gas cooled reactor, or 2) the fuel compacts are loaded directly into the insertion openings of the hexagon pillar type graphite block. Finally, the hexagon pillar type graphite blocks are loaded as the fuel into the reactor core while they are superposed one step on another step in a honey cam arrangement.
(Breakage of Fuel Compact)
In this case, when the treatment of the fuel compacts 10, that is when they are loaded into the fuel sleeve or the graphite block is carried out, the fuel compact 10 mechanically contacts with the inner surface of the fuel sleeve or the graphite block to thereby apply an impact onto the fuel compact 10 whereby the corner 10b of the fuel compact 10 is possibly broken (see FIG. 7).
In this manner, as a breakage arises in the fuel compact 10, a state of high temperature occurs within the high-temperature gas cooled reactor and therefore, when the fuel compact 10 is thermally expanded, the broken pieces thereof are held between the fuel compact 10 and the inner face of the fuel sleeve or the graphite block, which causes a high stress to occur in the place where the broken pieces are held, whereby the fuel compact 10, the fuel sleeve and the graphite block are damaged.
In addition thereto, since a temperature difference arises due to a difference of cooling efficiency between the central part of the high-temperature cooling gas cooled reactor and the peripheral part thereof when the fuel compact is used in the reactor and therefore the central part of the high-temperature gas cooled reactor has a temperature higher than the peripheral part thereof, the central part of the high-temperature gas cooled reactor has thermal expansion larger than the peripheral part thereof, with the result that the fuel compact 10 tends to be deformed into a drum-like shape. The thus drum-shaped fuel compact 10 causes its corner 10b to mechanically contact with the inner surface of the fuel sleeve or the graphite block, which causes the fuel compact 10 to be possibly cracked.
Thus, it is required to prevent such breakage of the fuel compact 10, but, in this case, it is also required to take a consideration of not damaging the coated fuel particles 12 by the stress when it is pressed.